1. Field of the Invention
This invention relates to a biaxially stretch blow-molded article and particularly to such an article having an improved base configuration and a bottom mold therefor.
2. Description of the Prior Art
As is well-known, biaxially stretch blow-molded articles such as containers made from polyethylene terephthalate resin (PET) are now in wide use, and are advantageous in their features such as good gas proofing, transparency, rigidity, and hygiene.
These containers are frequently used for so-called "hot-fill" applications where the beverage or liquid is dispensed in the container initially at a raised temperature, for the purpose of sterilization.
As the hot-filled liquid cools, it shrinks in volume reducing pressure in the container, and thus deforming a barrel portion of the container. To cope with volume shrinkage and pressure reduction, such containers have particular areas of the barrel portions deformed beforehand and a plurality of collapse panels are provided on the barrel portions thereof.
Such containers of a self-standing type have a bottom referred to as a "champagne bottom", in which a central inwardly concave dome is formed in an inner grounding bottom portion. The container having the champagne bottom is prone to a problem that its bottom is deformed by the hot-filled liquid for the following reasons.
Usually, the bottom is not adequately stretched and thinned down compared with the barrel portion. Although the bottom is stretched and oriented to a certain degree, molecules in the container resin are highly unstable at this point. Generally, if the container is not oriented and crystallized adequately, neither good resistance to heat nor mechanical strength can not be provided. Therefore, when the hot liquid is dispensed thereinto, the container tends to return to its original configuration, i.e. a preform configuration prior to the blow molding.
For the foregoing reasons, the bottom of container tends to buckle or deform due to a raised temperature and the weight of the hot-filled liquid.
To overcome the foregoing problem, there is proposed a central concave bottom in the shape of a staircase. Conventionally, annular circular flat walls are created at different levels from the grounding bottom portions at the central inwardly concave dome. (Japanese Utility Model Application Laid-Open No. 58-67719 proposes such a bottom profile.)
However, the central inwardly concave dome in the shape of a staircase increases its longitudinal stretch, but its latitudinal stretch is insufficient. Therefore, such a central inwardly concave dome is not oriented and crystallized latitudinally to a sufficient degree. Further, insufficient latitudinal stretch provides thick portions on one side of the bottom surface, and causes such thick portions on the bottom to be deformed extensively by heat. Though the bottom does not have good heat resistance uniformly on the entire region thereof, if a uniform thickness is provided in a circumferential direction, a deformation is substantially unnoticeable. However, partially thick portions of the bottom are prone to extensive deformation compared with thin portions, making the container aesthetically and commercially unacceptable.
Further, Japanese Patent Publication No. 2-36456 proposes a champagne bottom having concave portions and convex portions which extend radially of a central inwardly concave dome and which are alternately arranged circumferentially around the center of the bottom.
The champagne bottom of this configuration with deep concave and convex portions can be sufficiently stretched, so that it can maintain its aesthetic and commercial acceptability in spite of deformations due to uneven thickness. However, if the concave and convex portions are made deep, the formation of the bottom during the blow-molding is hindered. Since a sufficient amount of resin cannot be applied deep enough to the concave and convex portions, it is impossible to form such portions. To overcome this problem, a very high blow pressure should be applied.
When the blow pressure is raised, it is necessary to increase the mold clamping force. Therefore, a mold clamping mechanism becomes large in a molding machine, which would lead to a new problem, i.e. an extend, ed molding time. Further, it becomes necessary to prepare a complicated bottom mold for forming concave and convex portions on the bottom. This means that it becomes expensive to form the bottom since it takes time to prepare the bottom mold, and the number of manufacturing steps is increased